1. Field of the Invention
This invention relates to packet communication systems which communicate various communication information on voice, data, images, etc., and methods of controlling same, and more particularly to such a system which provides a transmission quality which satisfies both of high instantaneousness and low discard properties, and a method of controlling the system.
2. Description of the Related Art
As shown in a block diagram of FIG. 1, generally, a packet communication system which produces packets of information and transmits various pieces of information on voice, data images, etc., generated from various information terminals includes a high-speed data terminal 9-1, a low-speed data terminal 9-2, a voice terminal 9-3, etc.; terminal interfaces 10-1 to 10-3 corresponding to the respective information terminals; a line interface 12 to a multiplexing line 13; a high-speed packet bus 11 connecting the respective interfaces 10-1 to 10-3 and line interface 12. In this arrangement, the information generated by each of the terminals 9-1 to 9-3 is decomposed to predetermined information units by the corresponding one of the terminal interfaces 10-1 to 10-3, combined with an addressee information, etc., into a communication unit packet and input to the line interface 12 via the high-speed packet bus 11. The line interface 12 selects a multiplexing line 13 connecting to the addressee in accordance with information on the addressee in the input packet and transmits the packet to the multiplexing line 13 on a time-divisional basis.
If communication information generated by the respective terminals 9-1 to 9-3 concentrates in a given time in such packet communication system, so-called congestion of packets occurs in the communication system. In order to cope with this situation, a buffer is conventionally provided generally to store packets as a queue in the line interface 12.
In the buffer structure shown in FIG. 2(a), a buffer BF1 is provided to store and output received packets P.sub.1 -P.sub.4 in the order in which the packets are received. Another buffer arrangement, for example, shown in FIG. 2(b), includes a plurality of buffers BF.sub.1 -BF.sub.3, a demultiplexer DMPX which distributes received packets P.sub.1 -P.sub.4 in accordance with a priority of the packets, and a multiplexer MPX which selects an output packet in accordance with the priority of the packets. In this arrangement, for example, packets P.sub.1 and P.sub.4 of high priorities are stored in the buffer BF.sub.1 and transmitted preferentially while packets P.sub.1 and P.sub.3 of lower priority are stored in the buffers BF.sub.2 and BF.sub.3 in accordance with their priorities and are transmitted after the packets P.sub.1 and P.sub.4 of highest priority are transmitted.
The communication information handled by such a packet communication system is divided mainly into two: information which requires high instantaneousness and whose value itself would be impaired like the information on voice if transmission delay and/or reversal of packets with reference to time occurs; and information which requires a low discard property in order to prevent possible impairment to the meaning of the entire information, for example, caused by a failure of packets in data from data terminals, etc. Therefore, it is necessary to control inputting and/or outputting a packet queue so as to satisfy high instantaneousness and low discard properties of the information even if congestion can be avoided using buffers such as those as shown FIGS. 2(a) and (b).
In the buffer arrangement of FIG. 2(a), packets can only be output in the order of their reception, so that information of high instantaneousness cannot be output preferentially. In the buffer structure of FIG. 2(b), information of high instantaneousness is given high priority, so that it can be output preferentially to thereby ensure high instantaneousness. In such a system, however, information of lower priority increasingly tends to stay in the buffer to thereby cause an overflow and a problem of transmission delay exceeding an acceptable limit because information of higher priority is preferentially processed. In the buffer arrangement of FIG. 2(b), the sequence of packet outputting is determined only in accordance with priority irrespective of the number of packets in the priority-classified buffers. Therefore, if communication information concentrates in the buffer arrangement temporarily from the various information terminals, transmission delays of packets of lower priority would increase and packets would overflow easily. Possible packets arriving after the overflow would be discarded and thereby degrade their transmission quality.
As mentioned above, the prior art packet communication system avoids congestion of packets using one or more buffers, but selects the sequence of information outputting in accordance with priorities of the packets alone irrespective of the number of packets in the buffers. Therefore, transmission delay of packets of lower priorities increases and those packets are easily discarded to thereby cause the problem that a transmission quality having high instantaneousness and low discard properties cannot be obtained.